1. Field of the Invention
The invention pertains to logging vehicles of the type generally known as log skidders and particularly to a pressure control mechanism for the grapple mechanism mountable on the log skidder.
2. Description of the Prior Art
There are two general types of log skidders depending upon the boom configuration and its function. The rotary boom skidder has a boom movable through an arc permitting the operator to reach to the right and left of the vehicle to pick up trees that are not in the direct path of the vehicle. Other skidders employ a rigid boom which does not rotate but may be pivoted in a vertical plane to position the grapple over the end of the log. Only the grapple rotates and the machine must be driven to the tree. Either type of machine will usually have a winch driven cable passing over a fairlead on the boom with chokers on the end of the cable for skidding logs which cannot be reached by the grapple.
The grapple, mounted at the distal end of the boom, is lowered in its open position to engage and be closed about a group of logs lying on the ground for hauling by the skidder. The hydraulic cylinder powering the grapple is actuated to close the grapple about the group of logs and then the hydraulic cylinder actuating the boom raises the boom to permit the skidder to drag the logs along the ground to a central location.
It would be desirable to provide a method for maintaining pressure on the grapple cylinder of the grapple skidder since the gripping force of the grapple on the load of logs is directly proportional to the pressure in the grapple cylinder. With low pressure in the cylinder, the grapple will tend to drop logs.
Methods used to maintain constant pressure in the grapple cylinder include the installation of lock valves on the grapple cylinder to maintain pressure. A lock valve cancels out pressure loss due to leakage past a control valve spool, but cannot prevent pressure loss due to the internal cylinder leakage. A lock valve is also ineffective against pressure loss due to crushing of wood fiber or rearrangement of a load into a smaller geometric package.
The prior art also includes the provision of a nitrogen loaded accumulator and a lock valve in the hydraulic circuit controlling the grapple cylinder. Theoretically, this is a good solution, since the lock valve prevents leakage past the control valve spool and the accumulator provides a reservoir of high pressure oil to make up for other system losses. However, in practice, accumulators commonly lose their gas charge and it is impossible to visually determine if the accumulator is functional. Also, accumulators have some shipping restrictions against them due to their high pressure gas charge and they are a potential bomb in a fire.
Experiments with systems utilizing a special valve which detects low grapple pressure and shifts the control valve spool until the circuit is returned to some predetermined pressure have not proved fruitful.
The general failure of these conventional systems have caused most manufacturers to make no special effort to control grapple pressure. The skidder operator is relied upon to periodically actuate the control valve as the load is being skidded. Such a method of controlling grapple pressure is not reliable, depends upon the operator's skill, adds heat to the system and reduces the life of all hydraulic components in the system. Furthermore, the operator cannot steer while actuating the grapple valve.